1. Field of the Invention
The present invention relates to a machine tool such as, for example, a turning machine, a drilling machine, or a grinding machine and, more particularly, to the machine tool equipped with a measuring function for correction of, for example, a thermal displacement.
2. Description of the Related Art
In a machine tool such as, for example, a turning machine, thermal expansion and/or thermal deformation tend to occur at a machine bed and various parts of the machine tool because of a cutting heat and/or heat emission from such various parts incident to the machine tool being operated. The occurrence of the thermal expansion and/or the thermal deformation referred to above tends to bring about a reduction in processing accuracy. Although as a countermeasure therefor, a cooling device has been used, the cooling device tends to become too huge and bulky to suppress the thermal expansion sufficiently, and also, only cooling is insufficient to secure a processing accuracy. For this reason, various suggestions have been made for correcting, for example, the cut-in quantity of a tool depending on the thermal displacement by measuring the thermal expansion.
By way of example, as shown in FIG. 12, the machine tool disclosed in the Japanese Laid-open Patent Publication No. 2002-144191 is a turning machine having a structure in which a headstock 51 is fixed in position on a machine bed 52 and a tailstock 54 having a tool post 53 mounted thereon is supported to move in a spindle radial direction (X-axis direction). The illustrated turning machine is structured such that the position of the tool post 53 in the spindle radial direction is measured by causing a readout unit 56, fitted to the tailstock 54, to read a scale 55 fitted to the headstock 51 so as to extend in a direction parallel to the spindle radial direction. The reading indicative of the position of the tool post 53 in the spindle radial direction changes with, for example, thermal displacement. Accordingly, a proper processing accuracy is secured at all times by correcting the cut-in amount of the tool 57 of the tool post 53 in dependence on the measured reading.
If only the relative position of the tailstock 54 relative to the headstock 51 is measured such as disclosed in the Japanese Laid-open Patent Publication No. 2002-144191 listed above, an error tends to occur between a spindle axis and the tool 57 in the event of occurrence of the thermal displacement of the tool post 53 relative to the tailstock 54. Also, since in the machine tool disclosed in the Japanese Laid-open Patent Publication No. 2002-144191 referred to above, the headstock 51 and the tailstock 54 for the tool post 53 are arranged having been juxtaposed in a direction parallel to the spindle radial direction and have respective directions of spindle axes thereof (Z-axis directions) held at the same positional relation, the scale 55 fitted to the headstock 51 can be readout by the readout unit 56 fitted to the tailstock 54. In the case of the machine tool having a structure in which the tool post is movable in both of the spindle radial direction and the spindle axial direction, however, the positional relationship between the headstock and the tailstock in the spindle axial direction does not remain the same at all times, neither the scale nor the readout unit cannot be provided in such a machine tool as hereinabove described. This is also true of the machine tool in which the tool post is fixed in position, but the headstock is movable in both of the spindle radial direction and the spindle axial direction and also true of the machine tool of a structure in which the headstock and the tool post are movable in the spindle direction and the spindle axial direction, respectively.